In hybrid vehicles the engine is stopped when no torque is required from the engine to save fuel, reduce emissions and reduce noise. Current hybrid vehicles shut off the fuel supplied to the engine and let the engine come to stop without further control. However, following this method, the engine comes to a complete stop after a variable period of time depending on the engine temperature, speed, throttle position, etc.
Another drawback is that the engine acts like an air pump during engine shut down, pumping air into the catalyst that is stored in the catalytic converter. Air stored in the catalytic converter causes a NOx spike during subsequent startup of the engine. Another issue with uncontrolled engine stopping is the level of noise, vibration and harshness that results from engine shutdown.
This disclosure of engine control methods and systems is directed to solving the above problems and other problems as summarized below.